It is conventional to secure or fix weft thread ends by the formation of a so-called catch selvage along the weft entrance side of the fabric and along the weft exit side of the fabric. Each selvage is formed by at least two catch selvage threads acting as warp threads, so to speak, and the weft ends projecting from the loom shed. As soon as the catch selvage has served its purpose it is cut-off a short distance from the beat-up line of the fabric. A pull-off or withdrawal mechanism for the cut-off catch selvage cooperates with detour elements for withdrawing the cut-off catch selvage and transporting it into a collecting container. If the material flow of the cut-off catch selvage or selvages is not monitored and properly controlled, the weaving process may be disturbed by ripping of the catch selvages, by winding the catch selvages onto the catch selvage withdrawal apparatus or even by winding the cut-off catch selvages or one of these selvages onto the cloth take-up beam.
German Patent Publication DE 28 20 251 (Onishi et al.) discloses an apparatus for ascertaining a failure in the withdrawal of catch selvage yarns. It is not clear from the Onishi disclosure whether the monitoring takes place after the catch selvage yarns have been formed into a catch selvage or prior to such formation. In any event, a catch selvage yarn withdrawal mechanism (10, 12) including at least one driven roller cooperating with a further roller pulls the catch selvage yarn off and out of the loom. The catch selvage yarn or yarns is pinched between the two transport rollers which rotate in opposite directions so as to apply the necessary withdrawing force to the catch selvage yarns. At least one of the two rollers is spring-biased relative to the other roller to provide the necessary friction for the withdrawal. Further, at least one of the rollers is movable away from the other in response to any accumulation of selvage catch yarns between the two rollers in order to generate a fault or alarm signal when an excess of catch selvage yarns has been accumulated between the two rollers. The fault or alarm signal is produced when the normal spacing between the two rollers is exceeded by a predetermined value. Such a mechanism is not capable of accommodating different types and sizes of weft threads and catch selvage yarn qualities on the same loom, for example thick yarns in one weaving operation and relatively thin yarns in another weaving operation which are required to be handled by the same loom for different types of fabrics.
If after a certain yarn has been used in one weaving operation and thereafter different yarns are to be used, the known apparatus, according to Onishi et al. requires a fine readjustment of the withdrawal rollers, in order to assure a proper functioning of the known monitoring apparatus for indicating a fault in the withdrawal flow of catch selvage yarns. Another drawback of the known apparatus is seen in that its response characteristic is rather slow, especially when relatively thin catch selvage yarns and relatively thin weft threads are used because a certain accumulation of yarns between the transport rollers is required before the two rollers are sufficiently separated from each other to release a fault or alarm signal. Thus, the generation of a fault signal may not occur until substantial damage has already resulted.
European Patent Publication EP 0,681,044 A1 (Meyns et al.) discloses a method and apparatus for withdrawing cut-off catch selvages. A monitoring of the flow of the cut-off catch selvage is not disclosed. However, the withdrawal rollers are provided with a friction increasing surface to assure a proper withdrawal. Further, at least one of the withdrawal rollers is so positioned relative to the other withdrawal roller that a loop of more than 150.degree. is formed by the cut-off catch selvage around one of the rollers. However, such loop formation does not involve any monitoring to assure a proper withdrawal operation.